Aryloxide Ligands Research Articles

Monocyclopentadienyltitanium Aryloxide Complexes: Preparation, Characterization, and Application in Cyclization Reactions

A variety of monocyclopentadienyltitanium aryloxide complexes were prepared, characterized by X-ray crystallography, and used to catalyze or mediate cyclization reactions. Structural characterization allowed for the comparison of steric parameters of various 2,6-disubstituted aryloxide ligands. Transformations of dienes and enynesincluding the catalysis of a 1,6-diene cycloisomerization and the intramolecular Pauson−Khand reactionwere investigated. A titanium metallacycle was prepared from a sterically hindered enyne containing a trisubstituted olefin. The Pauson−Khand reaction of trimethylsilyl-substituted enynes was promoted to generate α-silylcyclopentenones.

Direct Synthesis of Heterometallic Europium/Barium Complexes: H2[Eu2Ba6O2(OiPr)16(THF)4] and EuBa2(OC6H4Me-4)7(diglyme)2(DME)

Convenient syntheses of mixed-metal Eu/Ba complexes starting from the elemental metals have been explored. A mixture of europium and barium ingots reacts in boiling 2-propanol to form a product which crystallizes from THF as the heterometallic complex formulated as H2[Eu2Ba6O2(OiPr)16(THF)4] (1). An analogous reaction in the presence of p-cresol (HOC6H4Me-4) forms trimetallic EuBa2(OC6H4Me4)7(diglyme)2(DME) (2), after recrystallization from THF/DME/diglyme (DME = 1,2-dimethoxyethane). Complex 1 exhibits an octametallic geometry comprised of two square pyramids with coplanar bases which share a basal edge and have apices on opposite sides. The six barium atoms occupy the basal positions and the two europium atoms are at the apices. Complex 2 exhibits a triangular arrangement of metal atoms with both terminal and bridging aryloxide ligands coordinated to the europium atom and bridging aryloxide and terminal ether ligands coordinated to the barium atoms.

Chelating Aryloxide Ligands in the Synthesis of Titanium, Niobium, and Tantalum Compounds: Electrochemical Studies and Styrene Polymerization Activities

The chelating trisphenol ligands tris(2-hydroxyphenyl)amine (1H3) and tris(2-hydroxy-4,6-dimethylbenzyl)amine (2H3) proved to be excellent precursors for the chelating phenoxides, and the latter has been used to prepare a series of cyclopentadienylmetal derivatives of early transition metals. For niobium and tantalum, reactions with CpMCl4 lead to the compounds CpMCl(1) and CpMCl(2). An X-ray diffraction study of CpNbCl(1) establishes a pseudo-octahedral structure with a trans disposition of the η5-cyclopentadienyl ring and the nitrogen atom of the chelating ligand. Similar reactions of CpTiCl3 lead to the CpTi(1) and CpTi(2) analogues. Electrochemical experiments provide useful information on the reduction potentials of the compounds, from which it is clear that ligand 2 is a stronger donor than is 1. At the same time, it appears that chelate ring size is important; while the reduction of complexes containing 1 are largely reversible, those of complexes containing 2 are irreversible. This is interpreted to mean that the six-membered rings in the latter are opening during reduction, a process involving formal loss of an aryloxide from the metal center. In an attempt to correlate this solution reactivity with catalytic efficiency in a bond-forming process, the compounds were screened for activity as styrene polymerization catalysts in the presence of methylaluminoxane cocatalyst. While the niobium and tantalum analogues were inactive, the titanium compounds of 1 showed high activity and appreciable selectivity for the preparation of syndiotactic polystyrene.

Synthesis and characterization of bis(indenyl) zirconium aryloxide derivatives and their use in α‐olefin polymerization

AbstractA series of new bis(indenyl) zirconium diaryloxides of general formula Ind2Zr(OL)2 (L = C6H5, 2; C6F5, 3; 2,6‐Me2C6H3, 4; 2,4,6‐Me3C6H2, 5; 4‐tBuC6H4, 6) were synthesized by a metathesis reaction of Ind2ZrCl2 (1) with the appropriate thallium aryloxide salt, TlOL. The complexes 1–6 were characterized by 1H and 13C NMR techniques. They were also examined as catalysts for ethene and 1‐hexene polymerization with methylalumoxane as co‐catalyst, and a trend of the polymerization activity as a function of aryloxide ligands was observed. An interpretation of this trend, considering both the electronic and steric effects of the substituents on the aryloxide rings, was proposed. Copyright © 2001 John Wiley & Sons, Ltd.

Synthesis and characterization of cyclopentadienyl titanium(aryloxide)sulfide complexes.

The role of sterically demanding aryloxide ligands in efforts to prepare terminal-sulfide derivatives of compounds of the form (η5-C5H5)‘Ti(OR)Cl2 are investigated. The LiCl adducts of the terminal-sulfide complexes (η5-C5Me5)Ti(OR)(S)·LiCl·(2,2‘-bipy) 8 and 9 are isolated. The compound (η5-C5Me5)Ti(OR)(S)(THF) 10 proved to be illusive as a result of its propensity for dimerization.

Rare Earth Complexes of Bulky 2,6-Diphenylphenolates Containing Additional, Potentially Buttressing 3,5-Substituents

The rare earth aryloxide complexes, [Yb(OArtBu)3(THF)]·THF, [Sc(OArtBu)3(THF)]·THF, [Yb(OArMe)3(THF)], and [Sm(OArPh)3(THF)2] (OArR = 2,6-diphenyl-3,5-di-R-phenolate), have been prepared by redox transmetallation/ligand exchange between the rare earth metal, bis(pentafluorophenyl)mercury, and HOArR in tetrahydrofuran. This reaction also provided [Yb(OArPh)3(DME)]·3/2THF by incorporation of adventitious 1,2-dimethoxyethane. A similar reaction between Yb metal, diphenylmercury, and HOArMe gave [Yb(OArMe)2(THF)3]. The homoleptic complexes [Yb(OArPh)3] and [Sc(OArH)3] have been prepared by direct reaction of the rare earth element with HOArR in a sealed tube in the presence of mercury at elevated temperatures (200−250 °C) and the latter was converted into [Sc(OArH)3(THF)] by treatment with THF. X-ray crystal structure determinations of [Yb(OArtBu)3(THF)]·THF, [Sc(OArtBu)3(THF)]·THF, and [Yb(OArMe)3(THF)] show the complexes to be monomeric and four coordinate with a trigonal pyramidal stereochemistry. These structures provide clear evidence that groups meta to the phenolate donor can modify the coordination behaviour of the 2,6-diphenylphenolate ligand. In [Yb(OArPh)3(DME)]·3/2THF a distorted square pyramidal stereochemistry is observed. The divalent complex [Yb(OArMe)2(THF)3] exhibits a distorted trigonal bipyramidal arrangement of the oxygen donor atoms with apical THF ligands. The homoleptic scandium 2,6-diphenylphenolate has three oxygen atoms in a triangular array with additional π-η1-Ph···Sc interactions above and below the ScO3 plane [C−Sc−C = 154.2(8)°], providing a new type of binding of this aryloxide ligand.

The isolation and chemistry of niobium and tantalum dimethylamides containing mono- and di-aryloxide ancillary ligands

The salt complex [Me2NH2][Ta(NMe2)2Cl4] 4 has been isolated from the reaction of [Ta2Cl10] with Me2NH and the anion shown to contain mutually cis-dimethylamido ligands. 4 reacts with Me2NH and pyridine to produce the neutral adducts mer,cis-[Ta(NMe2)2Cl3(HNMe2)] 1 (known compound) and [Ta(NMe2)2Cl3(py)] 5. The solid state structure of [Ta(NMe2)2Cl3(py-4Ph)] 7 shows a mer,cis arrangement of Cl and NMe2 groups. The addition of 2,3,5,6-tetraphenyl- or 2,6-di-isopropyl-phenol (2 equiv) to solutions of 1 in benzene was found to produce a mixture of two isomers each containing a residual Ta–NMe2 and Ta–NHMe2 group. These were formulated as [Ta(OAr)2Cl2(NMe2)(NHMe2)] with the coordinated amine trans to the Ta–NMe2 group and aryloxide ligands either mutually cis or trans. The cis isomer was found to thermally convert to the trans form. The trans isomers [M(OAr)2Cl2(NMe2)(NHMe2)] (M = Ta, OAr = 2,3,5,6-tetraphenyl- or 2,6-di-isopropyl-phenoxide; M = Nb, OAr = 2,3,5,6-tetraphenylphenoxide) are obtained in high yield by treatment of the corresponding tri(chlorides) [M(OAr)2Cl3] with excess Me2NH. The isomorphous/isostructural compounds [M(OC6HPh4-2,3,5,6)2Cl2(NMe2)(py)] (M = Nb, Ta) were structurally characterized and shown to contain the pyridine ligand trans to the Ta–NMe2 group with mutually trans aryloxides. Addition of 2,2′-methylenebis(6-phenylphenol) {(HOC6H3Ph)2CH2} to 1 resulted in formation of the compound [Ta{(OC6H3Ph)2CH2}Cl2(NMe2)(HNMe2)] 15. The solid state structure of 15 shows the nitrogen atoms to be mutually trans with cis aryloxide oxygen atoms. The eight-membered dioxametallacycle ring is puckered with the methylene bridge folded up towards the Ta–NMe2 group. The reaction pathways leading to these products are discussed.

Carbene transposition involving double dehydrogenation of an sp3 carbon

Two benzylic hydrogens of 2,6-Me2C6H3O− coordinated to RuCl(PCy3)2(CHPh)+ are transferred to the benzylidene ligand, liberating toluene to form a new carbene which is covalently linked to the aryloxide ligand.

Synthesis, Characterization, and One-Electron Reduction of Mixed-Cyclopentadienyl/Aryloxide Titanium Dichlorides

The synthesis and characterization of a series of titanium dichloride derivatives containing both cyclopentadienyl and various substituted aryloxide ligands, [CpTi(OAr)Cl2] 10−18 and [Cp*Ti(OAr)Cl2] 19, 20, have been investigated. Compounds 10−18 are formed from the reaction of [CpTiCl3] with either 1 equiv of the substituted parent phenol in the presence of excess pyridine or with 1 equiv of the corresponding lithium salt of the substituted parent phenoxide in hydrocarbon solvents. Compounds 13, 14, 16, and 17 are chiral molecules (a nonresolved racemic pair) as a result of the presence of either ortho-(1-naphthyl) or ortho-(1-tetrahydronaphthyl) substituents on the phenoxide ligands. Titanium dichloride compounds 19 and 20 are formed in the reaction of [Cp*TiCl3] with the corresponding lithium salts of the substituted parent phenoxides in hydrocarbon solvents. The compounds 11, 13, 14, 15, 16, 18, 19, and 20 have been subjected to single-crystal X-ray diffraction analysis, and each was shown to possess a pseudo-tetrahedral geometry. The compound [CpTi(OC6H2Np-2-But2-4,6)Cl2] (16) (Np = 1-naphthyl) reacts with 1 equiv of sodium (mercury amalgam) per titanium to produce the chlorine-bridged titanium dimer [(OC6H2Np-2-But2-4,6)CpTi{μ-Cl}2TiCp(OC6H2Np-2-But2-4,6)] (21). The solid-state structure of 21 shows that the cyclopentadienyl rings are arranged in a transoid fashion about the [Ti{μ-Cl}2Ti] core with a Ti−Ti distance [3.336(1) Å] and Cl−Ti−Cl bond angles [92.07(4)°] intermediate between those found in paramagnetic [Cp2Ti{μ-Cl}2TiCp2] and diamagnetic [(ArO)2Ti{μ-Cl}2Ti(OAr)2].

Syntheses and crystal structures of four-coordinate aryloxo neodymium complexes

Two four-coordinate complexes of neodymium with 2,6-di- tert-butyl-4-methylphenolate have been synthesized and their single-crystal structures determined. Anhydrous NdCl 3 reacted with ArONa in a 1:3 molar ratio in THF at room temperature to yield the neutral four-coordinate complex [(ArO) 3Nd(THF)]·MePh ( 1), which crystallized as a solvate, in which three aryloxide ligands and one THF ligand are coordinated to neodymium to form a slightly distorted tetrahedron. The reaction of NdCl 3 with 4 equiv. of ArONa in THF gave [(ArO) 4Nd][Na(THF) 6] ( 2). In this anion–cation pair compound, the neodymium and sodium atoms exhibit distorted tetrahedral and distorted octahedral coordination geometry, respectively. The average NdO(Ar) distance is 2.176 and 2.230 Å in complex 1 and 2, respectively.

Aryloxide ligand modification: new classical catalytic systems for olefin metathesis

The AM1-calculated partial oxygen charge of several phenoxide anions, potential ligands for tungsten-based classical catalytic systems for olefin metathesis, has been used as an indication of their electronwithdrawing ability. Based on the modeled set, a number of bis(aryloxide) derivatives of tungsten (VI) oxychloride have been synthesized by refluxing the parent phenol and WOCl 4 in toluene, and we have explored their ability to catalyze various metathesis applications. The studied complexes are precursors to active metathesis catalysts when heated in the presence of Bu 4Sn, and experimental conditions for the catalysis of ring opening metathesis polymerization (ROMP), acyclic diene metathesis (ADMET) polymerization, and ring closing metathesis (RCM) are reported. The catalytic ability of the studied complexes cannot be predicted based on a single structural or electronic parameter since ligand substitution also seems to affect other features such as solubility and chemical stability of the complex. Crystal data for bis(2,4,6-tribromophenoxy)tungsten (VI) oxychloride ( 7): Space group: P 1, triclinic. a=7.8459(5) Å, α=94.100(1)°, b=8.8504(5) Å, β=92.687(1)°, c=14.2994(9) Å, γ=95.359(1)°, V=984.64(10) Å 3, R 1=0.0258 and wR 2=0.0628.

2,6-Diphenylphenolates of calcium, strontium and barium exhibiting π-phenyl encapsulation of the partially naked cations

Solvent free 2,6-diphenylphenolates of calcium, strontium and barium have been prepared by direct reaction of the bulk metals with 2,6-diphenylphenol (HOdpp) at elevated temperatures in evacuated sealed tubes. Single-crystal X-ray diffraction studies of crystalline [Ca 2(Odpp) 2(μ-Odpp) 2]·(PhMe) ( 1), [Sr 2(Odpp)(μ-Odpp) 3]·(PhMe) ( 2) or [Ba 2(Odpp)(μ-Odpp) 3] ( 3), obtained by toluene extraction of the reaction mixtures, revealed different binuclear structural frameworks. In 1, both Ca atoms have one terminal and two bridging aryloxide ligands in a pyramidal array with the coordination vacancies occupied by π-bonded pendant phenyl groups (an η 3- and an η 1-Ph on one Ca and an η 6-Ph on the other). By contrast, 2 has unsymmetrical metal–oxygen coordination with one terminal and three bridging aryloxides attached to one metal and solely three bridging aryloxides bound to the other. There are additional intramolecular π-Ph–Sr interactions with three η 1-Ph groups on the four coordinate Sr and three η 2-Ph groups on the three coordinate Sr. In 3, the structure exhibits a similar metal–oxygen framework to 2, but there are an increased number of π-phenyl–M interactions (one η 2-Ph and two η 1-Ph on the four coordinate Ba and one η 6-Ph, one η 3-Ph and one η 2-Ph on the three coordinate Ba).

Synthesis and structure of niobium and tantalum compounds containing 2,6-diphenyl-3,5-di- tert-butylphenoxide ligation

A series of mononuclear derivatives of niobium(V) and tantalum(V) containing the new ligand 2,6-diphenyl-3,5-di-tert-butyl-phenoxide (OC 6HPh 2-2,6- tBu 2-3,5) have been isolated. The addition of the parent phenol (2 equiv. per Nb) to a benzene solution of [Nb 2Cl 10] leads to formation of the trichloride [Nb(OC 6HPh 2-2,6- tBu 2-3,5) 2Cl 3] ( 1), while treatment of the tetra-chlorides [Cp*TaCl 4] and [CpNbCl 4] with [LiOC 6HPh 2-2,6- tBu 2-3,5] leads to the mixed cyclopentadienyl, aryloxides [Cp*Ta(OC 6HPh 2-2,6- tBu 2-3,5)Cl 3] ( 2) and [CpNb(OC 6HPh 2-2,6- tBu 2-3,5)Cl 3] ( 3), respectively. Structural studies show a square pyramidal geometry for trichlorides 1– 3 with a basal aryloxide in each case. An axial aryloxide, Cp* and Cp ligand are present for 1, 2 and 3, respectively. The addition of HOC 6HPh 2-2,6- tBu 2-3,5 to the precursor [Ta(NMe 2) 5] results in formation of the mono-aryloxide [Ta(OC 6HPh 2-2,6- tBu 2-3,5)(NMe 2) 4] ( 4), which is also square pyramidal with a basal aryloxide ligand. Activation of a solution of 1 in benzene with 3 equiv. of nBuLi, followed by treatment with H 2 (1200 psi, 100°C) results in intramolecular hydrogenation of ortho-phenyl rings. Hydrolysis of the reaction mixture leads to isolation of 2,6-dicyclohexyl-3,5-di- tert-butylphenol ( 5), which has been structurally characterized.

Thermonanalytical Investigations of Monochlorobis (2,4-Pentanedionato) Vanadium(IV) Aryloxides

The thermal decomposition of the complexes [Vcl (acac)2(OAr)] (where acac=2,4-pentanedionato anion; OAr=–OC6H4O-M-4, OC6H4OBut-4) has been studied using non-isothermal techniques (DTA and TG). The TGA indicate that the substitution of chlorine in VCl2(acac)2 with aryloxide ligands results in an increase in the initial temperature of decomposition (IDT) of the new complexes. The role of the substituent at the aryloxide ring on the thermal stability of the complexes is depicted and hence described. The ultimate decomposition product in all the complexes has been identified as V2O5. The kinetic and thermodynamic parameters namely, the energy of activation E, the frequency factor A, entropy of activation S and specific reaction rate constant kr etc. have been rationalized in relation to the bonding aspect of the aryloxide ligands.

Ancillary aryloxide ligands in ethylene polymerization catalyst precursors

The compounds CpTiCl 2 (OC 6 H 3 - i -Pr 2 ) ( 1 ), CpTiCl(OC 6 H 3 - i- Pr 2 ) 2 ( 2 ), CpTi(R)(OC 6 H 3 - i -Pr 2 ) 2 (R= t -Bu 3 , s -Bu 4 , n -Bu 5 , Me 6 ) have been prepared and characterized. Compounds 1 or 2 in the presence of 500 equivalents of methylaluminoxane (MAO) act as catalyst precursors for ethylene polymerization. While the catalysts derived from the monocyclopentadienyl complexes are much less active that the metallocenes, there is a clear enhancement in the activity of about 40% as a result of the inclusion of a second aryloxide ligand. Reactions of 1 with AlMe 3 revealed stepwise formation of CpTi(Me)Cl(OC 6 H 3 - i -Pr 2 ) 7 and CpTi(Me) 2 (OC 6 H 3 - i -Pr 2 ) 8 , while subsequent addition of AlMe 3 afforded complete conversion to 8 , with formation of the aluminum species [AlMe 2 (OC 6 H 3 - i -Pr 2 )] n 9 . In contrast, the catecholate complex CpTi(O 2 C 6 H 4 )Cl 10 reacts with AlMe 3 yielding the paramagnetic species [CpTi(O 2 (C 6 H 4 ))·AlClMe 2 ] 2 11 . Incorporation of aryloxide ligands in modified metallocenes was readily accomplished with the preparation of Cp 2 ZrCl(OC 6 H 3 - i -Pr 2 ) 12 , Cp 2 ZrCl(OC 6 H 5 ) 13 , Cp 2 ZrMe(OC 6 H 5 ) 14 and Cp 2 TiCl(OC 6 H 3 - i -Pr 2 ) 15 . In combination with MAO, 12 , 14 and 15 effect the polymerization of ethylene with an 11% increase in activity over the parent metallocenedichlorides. The implications of the increased activity are considered. Crystallographic data are reported for 2 , 3 , 6 , 9 , 11 , 12 and 13 .

Cationic Group 4 metal alkyl compounds containing aryloxide ligation: synthesis, structure, reactivity and polymerization studies

Abstract A series of bis(alkyl) derivatives of titanium and zirconium [(ArO)2MR2] (M=Ti, Zr; R=Me, CH2Ph; ArO=various 2,6-di-substituted phenoxides) has been synthesized and their reactivity towards the Lewis acid [B(C6F5)3] examined. The benzyl compounds generate stable zwitterionic species such as [M(OC6HPh2-2,6-R2-3,5)2(CH2Ph)][η6-C6H5CH2B(C6F5)3] (M=Ti, R=H, 12; Me, 13: M=Zr, R=Me, 15). Structural studies of 12 and 15 show the boron anion π-bound to the metal center through the original benzyl phenyl ring. In contrast, treatment of the benzyl compound [Zr(OC6H3But2-2,6)2(CH2Ph)2] with [B(C6F5)3] leads to the cyclometallated compound [Zr(OC6H3But-CMe2CH2)(OC6H3But2-2,6)][η6-C6H5CH2B(C6F5)3] 16 which was structurally characterized. In contrast to this behavior the bis(methyl) species react with [B(C6F5)3] to produce unstable methyl cationic intermediates which decompose to a mixture of [Ti(OAr)2(CH3)(C6F5)] and [CH3B(C6F5)2]. The titanium zwitterionic benzyl compounds will react with alkynes and α-olefins to produce mono-insertion products such as [Ti(OC6H3Ph2-2,6)2{C(CH3)C(Ph)CH2(η6-C6H5)}][PhCH2B(C6F5)3] 24. In these compounds 1,2-insertion of olefins occurs followed by chelation of the original benzyl group to the metal center. Spectroscopic studies show the boron anion is non-coordinated to the metal center. Despite their thermal instability, the methyl cations can be generated in situ in the presence of olefins to produce polymers (ethene and propene) and oligomers (1-hexene). Studies show that the molecular weight of the polymers or oligomers increases systematically with the bulk of the aryloxide ligand. Spectroscopic studies of the polypropylene indicate 1,2-insertion is occurring with β-hydrogen abstraction to produce vinylidene end groups as the termination step.

Thermal studies of new precursors to indium–tin oxides for use as sensor materials in the detection of NO x

Control of combustion product emissions in both sub- and super-sonic jet engines can be facilitated by measurement of NO x levels with metal oxide sensors. In 2O 3, metal-doped SnO 2, and SnO 2 (as well as other materials) show resistivity changes in the presence of NO x but often their sensitivity, stability, and selectivity are low. This study was designed to develop new synthetic pathways to precursors that produce high purity, two phase In 2O 3–SnO 2. The precursors were formed by complexation of tin(IV) with chelating aryloxide ligands to give the ammonium salt, (NH 4) 2[Sn(ligand) 3]· xH 2O or the neutral molecule, [Sn(ligand) 2]· xH 2O, followed by reaction with In(III) to form the monomolecular precursors. Thermal studies of these precursors were carried out by thermal gravimetry (TG) and differential scanning calorimetry (DSC). Further studies by Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance spectroscopy (NMR) were also conducted.

Synthesis and structure of a mixed valence metal carbitoxide complex: Eu 3[O(CH 2CH 2O) 2CH 2CH 3] 4(OC 6H 3iPr 2-2,6) 3

The direct reaction of europium metal with HO(CH 2CH 2O) 2CH 2CH 3 followed by treatment with 2,6- iPr 2C 6H 3OH leads to the crystallographically-characterizable, mixed valent carbitoxide product Eu 3[O(CH 2CH 2O) 2CH 2CH 3] 4(OC 6H 3 iPr 2-2,6) 3 ( 1), which contains bridging alkoxide ligands and both terminal and bridging aryloxide ligands.

Reactions of Alkynes and Olefins with Tantalum Hydrides Containing Aryloxide Ancillary Ligation: Relevance to Catalytic Hydrogenation

The reactivity of the three hydride compounds [Ta(OC6H3Ph2-2,6)2(H)2Cl(PMe3)2] (1), [Ta(OC6H3Pri2-2,6)2(H)2Cl(PMe2Ph)2] (2), and [Ta(OC6H3But2-2,6)2(H)2Cl(PMePh2)] (3) toward olefins and alkynes has been investigated. The reactivity observed is highly dependent on the nature of the ancillary aryloxide ligands. The 2,6-diphenylphenoxide 1 reacts with styrene to produce 1 equiv of ethylbenzene and the styrene adduct [Ta(OC6H3Ph2-2,6)2(η2-CH2CHPh)Cl(PMe3)] (5). In contrast, 1 reacts with 3-hexyne to eliminate H2 along with formation of the analogous alkyne complex 6. Structural studies of 5 and 6 show a square-pyramidal geometry with an axial olefin (alkyne) unit lying along the Cl−Ta−P axis. Structural parameters support a tantalacyclopropane (tantalacyclopropene) bonding picture for these molecules. Compound 5 is converted back into 1 under H2 along with formation of PhEt. The dihydride 2 reacts with styrene to form 1 equiv of PhEt, H2, and the dehydrogenation product [Ta(OC6H3Pri-η2-CMeCH2)(OC6H3Pri2-2,6)Cl(PMe2Ph)2] (7). The related adduct [Ta(OC6H3Pri-η2-CMeCH2)(OC6H3Pri2-2,6)Cl(PEt3)2] (9) was isolated by treatment of [Ta(OC6H3Pri2-2,6)2Cl3] with PEt3/Bu3SnH and was structurally characterized. Labeling studies show that the H2 generated comes exclusively from the aryloxide o-Pri group which was dehydrogenated. Both hydrides initially attached to the metal are transferred to the olefin substrate. In the case of the 2,6-di-tert-butylphenoxide compound 3, reaction with styrene generates the mono-cyclometalated compound [Ta(OC6H3ButCMe2CH2)(OC6H3But2-2,6)(CH2CH2Ph)Cl] (9). Structural studies of 9 confirm the presence of a phenethyl group. The related trans-phenylvinyl compound 10 is produced when 3 is reacted with phenylacetylene. Addition of 2,6-dimethylphenyl isocyanide (xyNC) to 10 produces the bis(iminoacyl) derivative 11, in which xyNC has inserted into the cyclometalated carbon as well as the Ta−CHCHPh bond in 10. Structural studies of 11 confirmed the trans arrangement of the hydrogen atoms in the phenylvinyl group. Mechanistic studies of the formation of 10 and 11 show the presence of two competing pathways. The first involves direct elimination of H2 from the dihydride and formation of an intermediate olefin/alkyne adduct. The product then arises by CH bond activation of the aryloxide with the hydrogen transferring to a carbon atom of the tantalacyclopropane (tantalacyclopropene) ring. The second pathway involves insertion of olefin/alkyne into a Ta−H bond followed by CH bond activation by the remaining hydride.

Reactivity of Group 4 Metal Alkyl and Metallacyclic Compounds Supported by Aryloxide Ligands Toward Organic Isocyanides

The reactivity of the dimethyl compound [Ti(OC6H3Ph2-2,6)2Me2] (1) and titanabicyclic compounds [(ArO)2Ti(CH2CH(C4H8)CHCH2)] (6, 7) (formed by trans-coupling of 1,7-octadiene) toward organic isocyanides has been investigated. Dimethyl 1 reacts with 1 equiv of 2,6-dimethylphenylisocyanide (xyNC) to produce a cyclometalated product [Ti(OC6H3Ph2-2,6)(OC6H3Ph-η1-C6H4){N(xy)CHMe2}] (2), which was structurally characterized. Compound 2 is believed to arise via CH bond activation within an intermediate η2-imine (azatitanacyclopropane) formed by transfer of both methyl groups to the isocyanide. With an excess of xyNC, 1 produces an intermediate bis(η2-iminoacyl) 4, which slowly converts to the enediamide [Ti(OC6H3Ph2-2,6)2{N(xy)CMeCMeN(xy)}] (5). The solid-state structure of 5 shows the presence of a folded diazatitanacyclopent-3-ene ring formed by intramolecular coupling of the two iminoacyl groups. Titanabicyclic compounds 6 and 7 react with ButNC and xyNC respectively to generate products 8 and 9, which both contain amide ligands derived by CH bond activation and an iminoacyl group formed by insertion into the new Ti−C bond. The solid-state structure of [Ti(OC6H3Ph2-2,6){OC6H3Ph-(η2-(C6H4)CNBut)}{ButNCH(C8H14)}] (8) shows a trans-fusion between the five- and six-membered carbon rings as found in the initial titanabicycle 6. The treatment of 1 with xyNC in the presence of excess 3-hexyne or styrene produces azatitanacyclopent-2-ene 10 and azatitanacyclopentane 11 derivatives, respectively. These arise via coupling of the alkyne or olefin with an intermediate η2-imine. The solid-state structure of [Ti(OC6H3Ph2-2,6)2{N(xy)C(Me2)C(Et)C(Et)}] (10) confirms its formulation.